Fix HTML structure and emphasize pyramid volume calculation

Corrected HTML structure and added strong tag for emphasis.

Author: Gmac4247

index.html

@@ -4296,9 +4296,10 @@ <h3 style="margin:7px">Volume of a pyramid</h3>
   <img class="center-fit" src="tetraFrame.jpeg" alt="figure-Tetrahedral-frame-on-circular-base" >
   </div>
 <br>
-<p style="margin:12px">The volume of a pyramid can be calculated 
+<strong style="margin:12px">The volume of a pyramid can be calculated 
 with the same coefficient as the volume of a cone.
-</p>
+</strong>
+<br>
 <br>
 <math style="margin:12px" xmlns="http://www.w3.org/1998/Math/MathML">
 <mrow>